Two causal mechanisms of neurodegeneration that are found in Alzheimer's disease (AD) and AD-related diseases such as amyotrophic lateral sclerosis-frontotemporal dementia (ALS-FTD) are re-initiation of the mitotic cell-cycle in neurons, and impaired nucleocytoplasmic trafficking resulting from disruptions in the nuclear envelope (NE) and nuclear pore complex (NPC). Deeper insight into how these abnormalities arise would clarify approaches to design effective treatments for these diseases; however, very little is known about the initiating mechanisms involved. This application will test the hypothesis that neuronal quiescence is maintained throughout life via active mechanisms that inhibit the mitotic cell-cycle and that re-initiation of the cell-cycle leads to NE/NPC disassembly, a normal occurrence in mitotic cells. This hypothesis is based on our study of the six-transmembrane enzyme GDE2 (Glycerophosphodiester phosphodiesterase 2; GDPD5), which cleaves the GPI-(Glycosylphosphatidylinositol)-anchor that tethers some proteins to the plasma membrane. GDE2 is a potent inhibitor of the mitotic cell-cycle and induces the differentiation of mitotic progenitors into post-mitotic neurons in the developing nervous system. We discovered that in adult mice lacking GDE2 (Gde2 KO), cortical neurons show evidence of cell-cycle re-entry, suggesting that GDE2 is required to preserve neurons in a quiescent state. Strikingly, Gde2 KO neurons that have re-entered the cell-cycle show abnormal organization of the NE, aberrant distribution of NPC proteins and impaired nucleocytoplasmic transport, raising the possibility that cell-cycle re-initiation and NE/NPC breakdown are linked. Consistent with this idea, genetic reduction of cyclin D, a critical regulator of the G1/S transition, suppresses nucleocytoplasmic transport-dependent neurodegeneration in a Drosophila model of c9ORF72 ALS-FTD. Notably, Gde2 KO mice display age-progressive neurodegeneration and GDE2 distribution and function is disrupted in AD patient neurons. These observations suggest that maintenance of neuronal quiescence is an active process and that failure of this process re-initiates the cell-cycle, triggers NE/NPC breakdown and induces neurodegeneration. Aim 1 will determine if GDE2 encodes a new pathway that maintains neuronal quiescence and will determine if neuronal cell-cycle re-entry signals NE/NPC breakdown in neurons. Preliminary RNAseq, analysis of Wnt-reporter mice in Gde2 KOs, and genetic studies in Drosophila identify aberrant activation of canonical Wnt signaling as a candidate pathway that induces neuronal cell-cycle re-entry, NE/NPC breakdown and neurodegeneration. Aim 2 will utilize mouse and Drosophila models to test this hypothesis. Studies in Aim 3 will determine links between GDE2 dysfunction, neuronal cell-cycle re-entry and NE/NPC breakdown in disease using Drosophila models of AD and ADRD, human postmortem tissue and iPS human neurons. These studies will provide new molecular insight into cross-disease triggers of neurodegeneration important in human AD and ADRDs.